Honeycomb has long been known for use in structural elements where a combination of lightness and strength are desirable. The use of nonmetallic materials such as ceramics and plastics offers additional advantages in terms of processing characteristics and the appearance and properties of the final product.
Thermoset plastics have been used in the manufacture of honeycomb, including honeycomb formed by bonding together corrugated ribbons of the plastic as well as honeycomb formed by bonding together corrugated ribbons of fabric impregnated with thermosetting resin followed by curing the resin in the honeycomb. Honeycomb structures formed with thermoset plastics have certain disadvantages, however, and these include limitations on the manufacturing techniques and a low resistance to damage. Because of the low damage resistance, the product is highly susceptible to damage during fabrication and lay-up and to fragmentation upon handling.
The present invention provides plastic-based honeycomb which does not suffer these disadvantages, but instead has acceptable mechanical properties and the capability of being fabricated and handled by simpler and more versatile techniques. Honeycomb structures in accordance with this invention have the strength, impact resistance and processing characteristics of thermoplastic materials and yet offer high temperature resistance and solvent resistance characteristics which permit such processing steps as the application of multiple coatings for purposes of density adjustment and the formation of composite layers.
Central to the invention is the selection of a linear prepolymer which is soluble in a preselected solvent, and which, without crosslinking, is heat curable to a linear polymer which is insoluble in that solvent, the cured polymer having a glass transition temperature equal to or greater than about 200.degree. C. A fiber web is initially impregnated with the prepolymer, using any of a variety of techniques, to bond the fibers of the web into a stable coherent structure. A laminated structure which is expandable into a honeycomb panel is then formed by arranging ribbons or sheets of the impregnated web into a stack, with adjacent ribbons or sheets bonded together along transverse node lines. The laminated structure is then expandable into an open-cell honeycomb structure, with the optional application of heat to facilitate the expansion. The expanded structure is then heated further to complete the cure and stabilize the structure. The cured resin has a thermoplastic character which permits expansion and handling without damage to the resin. The insolubility of the cured resin permits the resin to be applied in stages, with repeated applications subsequent to the expansion and initial cure if desired. In this manner, high resin loadings can be achieved, and layers of different resins or coating materials can be applied to form a composite structure with a specified combination of properties. In preferred embodiments, the initial impregnation is done by dipping the fiber web in a solution of the prepolymer.